When a vehicle is traveling on a slippery road such as a frozen road or a snow-compacted road, the driver's attempt to accelerate the vehicle by stepping on the accelerator may abruptly raise the rotating speed of the drive wheels and cause the drive wheels to spin out. Conversely, the driver's attempt to decelerate the vehicle by stepping on the brake may abruptly lower the rotating speed of the drive wheels and cause them to lock (in the following description, these phenomena will be collectively referred to as a slip). When such a slip occurs, the behavior of the vehicle is destabilized. With steering out of control, it is difficult for the vehicle to travel stably. Thus, it is important to reliably suppress the occurrence of such a slip.
One conventional method for detecting the occurrence of a vehicle slip involves detecting the wheel speed of the drive wheels and that of the idler wheels, computing the slip ratio of the drive wheels based on the detected wheel speeds, and determining whether the slip ratio exceeds a determination value so as to detect whether a slip has occurred. Another method involves detecting the wheel speed of the drive wheels and that of the idler wheels, computing the difference between the wheel speeds, and determining whether the speed difference exceeds a determination value in order to detect whether a slip has occurred (see JP-2002-27610-A, among others).